1. Field of the Invention
The present invention relates to a fiber grating based apparatus for detection of wavelength shifts. More particularly, this invention relates to a device using a pair of tuned fiber gratings to track wavelength shifts in wavelength-encoded fiber sensors.
2. Description of the Related Art
Recently, in-fiber grating sensors have attracted considerable interest because of their intrinsic nature and wavelength-encoded operation. These sensors are expected to be useful in a variety of applications such as in the area of advanced composite materials or "smart structures." In such applications, fibers can be embedded or incorporated into the materials or structures, without compromising the integrity of such materials or structures, to allow for the real time evaluation of conditions such as load, strain, temperature, and vibration to which the materials or structures are subjected.
Fiber gratings are well suited for use as sensing elements. When a fiber grating is illuminated, the grating reflects a narrow band of light at a specified wavelength. However, a measurand (e.g. strain, temperature) will induce a perturbation of the grating sensor which changes the wavelength of the light reflected by the grating. The value of the measurand is directly related to the wavelength reflected by the grating and can be determined by detecting the wavelength of the reflected light. This wavelength-encoded nature of the output of fiber gratings has advantages over intensity based sensing techniques because of the self-referencing nature of the output. The sensed information is encoded directly into the wavelength, which is an absolute parameter, and does not depend upon total light levels, losses in fibers or couplers, or variations in source intensity. Intensity based sensing schemes, on the other hand, depend upon total light levels and are affected by losses in the connecting fibers, by losses in couplers, and by variations in source intensity.
While the self-referencing nature of wavelength-encoded sensors makes the sensors well suited for use in advanced composite materials or structures, conventional approaches to accurately detect and determine the wavelength shifts of sensor returns are not well suited for such uses. Conventional means, such as a spectrometer, a monochromator, or use of a dispersive element coupled with an image array, are unattractive due to their cost, bulk optical nature and size, weight, and lack of ruggedness. Attempts to address these issues such as scanning a range of wavelengths with a single detector or using an interferometer as a wavelength detector are also unattractive because these attempts do not provide instantaneous or continuous detection and determination of wavelength. Additionally, they are difficult to incorporate into a system with several sensors in a single fiber without using time division multiplexing techniques.